The mechanical losses such as a bearing friction loss and a brush friction loss account for an increasingly large portion of the internal loss of a rotor for an electric generator or motor as the size of the rotor gets smaller. Therefore, smaller rotors having a diameter of 5 mm or less often consist of permanent magnet rotors that do not required brushes. The rotor shaft for such a rotor is required to have a certain diameter that is necessary for transmitting the load or drive torque. It is therefore conceivable to use a support shaft that is passed centrally through a cylindrical permanent magnet as disclosed in Japanese patent laid open publication No. 2000-130176. This is suitable for achieving a high mechanical strength in the connection between the shaft and permanent magnet. However, according to this structure, the cross sectional area of the permanent magnet diminishes by the cross sectional area of the support shaft, and the axial dimension of the permanent magnet is required to be increased to ensure an adequate magnetic flux required to produce an adequate torque without increasing the outer diameter of the permanent magnet.
To overcome such a problem, it is possible to press fit a cylindrical permanent magnet into a sleeve, fit bearing support portions into either axial ends of the sleeve, bond the bearing support portions with the corresponding axial ends of the permanent magnets that are recessed from the corresponding axial ends of the sleeve, and weld the peripheral part of each bearing support portion to the corresponding annular edge of the sleeve, as disclosed in Japanese patent laid open publication No. 11-234975.
However, according to the structure disclosed in Japanese patent laid open publication No. 11-234975, because the rotor is indirectly connected to the drive source consisting of a turbine via the bearing support portion, not only the axial dimension cannot be reduced but also a significant amount of eddy current loss is inevitable because the sleeve is made of metallic material even though it may not be a magnetic material.